Xperia Z5 :: Strange Noise When Music Stops

I have a Xperia Z5 Compact
 
When I have my phone plugged into an audio jack cable and play music through an amplifier everything is fine until the music stops
playing; either because the album is over or the playlist is over and then I get a thumping electronic noise. It comes in spaces like dut dut dut dut dut dut.
 
It happens a few seconds after the phone has been inactive and the phone has gone standby, it stops when I unlock the screen or just activate the phone. Then If I wait until the phone goes inactive again I get the same noise.